1. Field of the Invention
The present invention relates to improvements in monitoring and controlling substation power distribution. Specifically, the invention involves a method and apparatus for automatically monitoring substation power distribution and usage and transmitting the information to a remote location in order to provide real time, improved-accuracy monitoring of consumer loads and to control a remote power distribution substation from a remote location.
2. Description of the Related Art
At each power substation, there are generally transmission and distribution circuits, transformers, circuit breakers, capacitor banks, and other electrical equipment (hereinafter "power equipment") which deliver electrical power. The various pieces of electrical equipment, including the circuits and transformers, are protected for overload and fault conditions by monitoring the current in each phase of the equipment. The normal method of monitoring this current is by using a current transformer. A current transformer transforms the primary current (normally in the range of 100 to 3000 amps) to a secondary current in the range of 5 amps. The secondary current is then monitored via protection equipment, and upon detection of an overload or fault, the protection equipment will trip (open) the appropriate breakers to terminate the fault.
Because the current transformer has a known primary to secondary ratio, the current transformer can also be considered as a current sensor. This current sensor is used via a transducer in many substations to provide a DC signal which is representative of the primary current. In most applications today, the transducer is installed by inserting it in series with the secondary of each current transformer which is to be monitored.
Another possibility for monitoring the secondary current is to pass the secondary lead of the current transformer through a solid core current transformer, and measuring the output of the solid core transformer.
If the substation is being retrofitted for collection of this current data, the secondary lead of the current transformer must be cut and wired through the transducer, or at least disconnected and passed through a solid core current transformer.
Even though the secondary lead of the protection circuit current transformer only carries approximately 5 amps, if this wire is severed, the voltage across the open ends will increase to whatever voltage is necessary to provide a current in the secondary lead of the current transformer corresponding to the current flowing in the primary side of the current transformer. This voltage could become very high, creating a dangerous condition, including developing an arc within the current transformer. Therefore, in order to install a transducer in the current transformer secondary wire safely, the entire substation, or a portion of the substation, is de-energized so that the current transformer secondary wire may be cut and the transducer installed without serious danger.
This process is very time consuming and expensive. For instance, if a substation had ten circuits, each with three phases, it would take approximately 40 man hours to install the transducers in all of the protection circuit lines.
Moreover, in order to repair the transducer, it is often necessary to shut down the substation, or at least a portion of the substation.
Another disadvantage to present systems is that the meters connected to the transducers are located at the substations. Therefore, in order to monitor the power usage through the substation, a person reads the meters and records the measurements. This does not provide real time information. Thus, this type of monitoring generally results in providing more power through a station to handle the maximum expected load than is absolutely necessary.
Systems, such as those disclosed in U.S. Pat. Nos. 4,396,915 and 4,315,251, disclose automatic meter reading and control systems. The systems disclosed automatically read meters at the point of use by individual consumers and transmit the data to central location. These systems, if applied to the power substation contexts, would provide real time data; however, these systems would not solve the problem of cutting into the protection circuit at a substation to install the transducers which operate the meters.